It is known in the art of smelting and purifying metals to introduce gas into molten metal to remove impurities. Specifically, when processing molten aluminum, it is desirable to remove dissolved gases, particularly hydrogen, or dissolved metals, particularly magnesium. Those skilled in the art refer to removing dissolved gas from molten aluminum as "degassing", and refer to removing magnesium as "demagging." Nitrogen or argon is generally released into molten metal for degassing while chlorine gas is generally used for demagging. The present invention is particularly directed to the process of demagging, although it can also be used for degassing.
When demagging or degassing aluminum, chlorine or nitrogen gas, respectively, is released into a quantity of molten aluminum, this quantity generally being referred to as a bath of molten aluminum. The bath is usually contained within the walls of a reverbatory furnace. When demagging aluminum, chlorine gas is released into the bath and the chlorine bonds, or reacts, with the magnesium wherein each pound of magnesium reacts with approximately 2.95 pounds of chlorine to form magnesium chloride (MgCl.sub.2). Several methods for introducing chlorine into a molten aluminum bath are disclosed in the prior art.
For example, U.S. Pat. No. 3,650,730 to Derham et al. discloses introducing a flux, rather than chlorine gas, into molten aluminum to demag the aluminum. The flux contains a double salt of chlorine, such as Cryolite. U.S. Pat. No. 3,767,382 to Bruno et al. discloses an apparatus whereby chlorine gas is introduced through a rotating hollow shaft and impeller arrangement into the center of a pump chamber contained within the molten aluminum. U.S. Pat. No. 4,169,584 to Mangalick discloses a gas-injection system including a pump, a metal-transfer conduit and a gas-injection conduit connected to the top of the metal-transfer conduit. In the Mangalick disclosure, molten aluminum is pumped through the metal-transfer conduit and gas is injected into the upper portion of the pumped molten metal moving through the conduit. In practice, the actual product made by the assignee of the Mangalick patent has a gas-injection conduit connected to and extending through the top of the metal-transfer conduit into the upper portion of the pumped molten metal. As the molten metal moves past the submerged end of this gas-injection conduit, chlorine gas is introduced into the stream through a hole in the bottom of the gas-injection conduit.
U.S. Pat. No. 4,351,314 to Koch discloses a molten metal pump and gas-injection apparatus. The pump includes a pump casing having an inlet and an outlet port. An impeller is enclosed by the pump casing. The gas-injection apparatus comprises a tube having a first end connected to a gas source and an output end positioned within the molten metal bath, the output end being connected to a collar mounted on the pump casing, wherein the collar has a passage that communicates with the inlet. Gas is introduced through the tube into the passage and is released into the molten metal entering the inlet.
U.S. Pat. No. 4,003,560 to Carbonnel discloses a gas-treatment device comprising a purification device, which is immersible in a molten metal bath contained within a furnace, and a decanting and degassing tank located outside of the bath. Gas is introduced via a pipe into the purification device and the molten metal is pumped from the purification device into a decanting and degassing tank. The gas is then separated from the molten metal and the purified molten metal is drawn from the tank by a spout.
One problem with the prior art devices is that the chlorine gas is usually introduced into the molten metal near, or in an area enclosed by, machinery or equipment, particularly pumping equipment, which tends to rapidly clog and bind the equipment because the MgCL.sub.2 formed in the demagging process can adhere to equipment surfaces. For example, in the previously-described Mangalick device, chlorine gas is introduced into a metal-transfer conduit via a gas-injection conduit. The magnesium and chlorine react inside of the conduit and form MgCl.sub.2, which can adhere to the surface of the conduit and eventually clog it. This often occurs during start-up periods when the metal is cool and its flow rate is low. The Mangalick device is especially prone to clogging because: 1) the gas is released through a single, relatively large opening, 2) the opening is formed at the end of a relatively wide conduit, and 3) the conduit extends into the molten metal stream from the top of the metal-transfer conduit. As the pumped metal moves past the conduit, a low-pressure zone is created behind the conduit. The injected gas exits the gas-injection conduit and immediately enters the low pressure zone behind the conduit. There, it rises until it contacts the inner surface of the top of the metal transfer conduit. A large percentage of gas injected using this device contacts the inner surface and remains in contact with this surface until it exits the metal-transfer conduit. Magnesium chloride, therefore, tends to form along this surface.
Some other known gas-injection pumps, such as the previously described Koch device, introduce chlorine gas at a location within the molten metal bath where the gas can enter the pump chamber. The chlorine gas bonds with magnesium to form MgCl.sub.2 and the MgCl.sub.2 can bond to equipment surfaces thereby clogging the pump chamber or the outlet port, or binding the impeller.
Another problem with the prior art devices is that their efficiency is relatively low, the demagging efficiency being measured by the percentage of chlorine introduced into the molten metal that actually bonds with magnesium to form MgCl.sub.2. The efficiency of the prior art devices is low generally because: 1) the gas is not introduced into a pumped molten metal stream, but instead into relatively slow-moving molten metal, sometimes at a position where gravity is moving the molten metal through a restricted opening or conduit between two chambers, 2) the gas is introduced into or near a pumped molten metal stream but is introduced at a location where the gas is not dispersed throughout the stream and/or is not contained within the stream for a long enough period, and 3) the gas is introduced in large bubbles, which have a relatively small surface area, as compared to smaller bubbles, for a given quantity of gas.
Even a device that confines the chlorine gas and molten metal in an enclosed area, such as the one described in U.S. Pat. No. 4,169,584 to Mangalick, which confines the chlorine gas and molten metal stream within a metal-transfer conduit, is relatively inefficient. As previously described, this device includes a gas-injection conduit that extends through the top of a metal-transfer conduit, the part of the gas-injection conduit that extends into the metal-transfer conduit having an outside diameter of approximately 15/8"-2". When the molten metal stream moving through the metal-transfer conduit contacts the gas-injection conduit, it is obstructed by and diverted around the gas-injection conduit creating a low pressure zone behind the gas-injection conduit. At least some of the gas released through the bottom opening in the gas-injection conduit immediately enters the low pressure zone and quickly rises to the inner surface of the top of the metal-transfer conduit and is not swept into the moving stream. Therefore, the gas is not well dispersed within the stream and interaction between the gas and the molten metal is limited. As it will be appreciated by those skilled in the art, the greater the dispersion of gas within the molten metal stream the greater the demagging efficiency because the gas molecules contact a higher number of metal molecules, thus giving more molecules the chance to interact and bond to form MgCL.sub.2.
In the Mangalick device, and other known devices, the interaction between the gas and the molten metal is further limited because the gas is introduced into the molten metal through a single opening approximately 1/2" to 3/4" in diameter. As gas is released through this relatively large opening, large gas bubbles are formed. As explained previously, a given quantity of gas introduced into the molten metal as large bubbles does not have as great of an overall surface area as the same quantity of gas introduced into the molten metal as smaller bubbles. As it will be understood by those skilled in the art, the greater the surface area of the gas interfacing with the molten metal, the greater the demagging efficiency. Furthermore, small bubbles are more easily dispersed throughout the molten metal stream.
Improving the efficiency of the demagging process reduces material costs because less chlorine gas is used. Furthermore, chlorine gas that does not bond with magnesium either bonds with aluminum to form aluminum trichloride or rises to the top of the molten metal bath and escapes into the atmosphere, where it is an undesirable pollutant. A higher efficiency reduces the amount of chlorine gas released into the atmosphere.
Additionally, the known gas-injection or gas-release devices do not lift, or transport, molten metal to the surface. These devices generally release large bubbles that are not well dispersed within the flowing stream and the large gas bubbles simply rise through the molten metal to the top of the bath rather than lifting a portion of the molten metal steam upward. Therefore, the surface of the bath usually has a solid crust of metal and impurities on it. When scrap metal is placed in the bath, it often will rest on the crust and not sink into the molten metal where it would melt and be recycled. To solve this problem, circulation pumps or other devices are used to circulate the molten metal and melt the crust. A device that would melt enough of the crust to allow scrap to sink, without the added expense of a circulation pump, would save the cost of the circulation pump and the cost of maintaining it.